ASN.1 has an specific string type, called NumericString (tag 18). The
value of this type can be numeric characters (0-9) and space.

Fixes #22396

Change-Id: Ia6d81ab7faa311ff22759bf76862626974d3013e
Reviewed-on: https://go-review.googlesource.com/78655
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>